1. Field of the Invention
The present invention relates to a ceramic envelope made of a light transmission ceramics used for a high intensity discharge lamp such as a high pressure sodium lamp or a metal halide lamp.
2. Description of Related Art
Conventionally, an electric discharge lamp using a ceramic envelope made of a light transmission ceramics whose electric discharge light emitting space is formed in a cylindrical shape is widely used as a ceramics metal halide lamp. Such a ceramic envelope has a structure as shown in FIG. 5A and FIG. 5B, i.e., a cylindrical barrel section 10 forming an electric discharge light emitting space at its center. Its left and right ends are closed by an annular closing material 11, and its structure is provided such that a capillary tube 12 is connected in an orientation opposite to each other at the substantial center of the closing material 11. Then, after a light emitting substance or startup gas is sealed in an electric discharge light emitting space, an electrode is inserted into the capillary tube 12, and is sealed, thereby forming an electric discharge lamp.
These sections such as the barrel section 10, closing plate 11, and capillary tube 12 are each molded independently, and are integrally coupled with each other, thereby forming a ceramic envelope.
In the above ceramic envelope having the cylindrical electric discharge light emitting space, a temperature at a corner part of a cylinder is the lowest during lighting. Thus, a light emitting substance that is a corrosive substance easily accumulates at that site. As a result, though a light color change in the lighting direction is reduced, corrosion at the corner is easily advanced because the light emitting substance easily accumulates. The corrosion at that portion determines a service life of the lamp.
In addition, in such a ceramic envelope having a cylindrical electric discharge space, there is employed a fabrication method in which plural parts are assembled and bonded due to contraction action during burning. In another case as shown in FIG. 5A which is a partially enlarged view of the container, a wedge shaped cavity 13 is easily formed at a bonding section. In particular, a light emitting substance easily enters the cavity 13 generated at the corner of the barrel section 10, which causes corrosion, and impairs extended service life. Further, such a wedge shaped cavity easily causes stress concentration, and extended service life is impaired due to a thermal stress generated during lighting.
In addition, when the barrel section 10 is bonded with a double-ended closing material 11, as shown in FIG. 5A, the barrel section is gradually converged in the vicinity of its center. As shown in FIG. 5B, a constant diameter is maintained in the vicinity of the center of the barrel section, however, the diameter is changed or increased at the corner of an electric discharge light emitting space. In any case as well, an inner diameter D5 at the center of the barrel section is always smaller than an inner diameter D6 of an end.
This difference of the diameters is because a distance between an electric discharge arc and the barrel section during lighting is required to maintain a predetermined value due to restriction on material strength. Therefore, the inner diameter of the end was set much greater than its need when the inner diameter of a light emitting section was set with the arc and the vicinity of the center of the barrel section being a reference. As a result, the coolest point temperature is, of course, low. Thus, a light emitting substance easily accumulates, and corrosion is accelerated, which causes impairment of the extended service life.
Further, the linear transmission rate of a ceramic envelope depends on the surface roughness Ra, and thus, the smaller Ra is advantageous. Although the surface roughness of the interior surface of the ceramic envelope can be controlled by means of polishing, the process becomes complex, which is not rational. In addition, MgO or La2O3 and the like which are weaker with respect to a relevant halide than alumina mixed as an additive, appears on the interior surface by such polishing. Thus, there has been a disadvantage to maintain good electric discharge characteristics.
The present invention has been made in view of the foregoing problem. It is an object of the present invention to provide a ceramic envelope for high intensity discharge lamp that makes it possible to extend service life of the lamp even if an electric discharge light emitting space is formed in a cylindrical shape.
According to a first aspect of the present invention, there is provided a ceramic envelope for high intensity discharge lamp made of a light transmission ceramics, comprising:
a cylindrical barrel section forming an electric discharge light emitting space;
an annular closing section that closes both ends of the barrel section, respectively;
a capillary section for inserting and fixing an electric discharge electrode to be outwardly protruded so as to be opposed to each other from a substantial center position of both closing sections,
wherein the barrel section thickness of at least one of the boundary sections between both of the barrel section and closing section is continuously increased at a ratio from 1.2 to 2.0 relevant to the thickness in the vicinity of the center of the electrical discharge light emitting space.
With this construction, even if a light emitting substance accumulates at an electric discharge space corner, a time required predetermined amount of corrosion reduction can be prolonged, and service life of the lamp can be extended.
According to a second aspect of the present invention, there is provided a ceramic envelope for high intensity discharge lamp made of a light transmission ceramics, comprising:
a cylindrical barrel section forming an electric discharge light emitting space;
an annular closing section that closes both ends of the barrel section, respectively;
a capillary section for inserting and fixing an electric discharge electrode to be outwardly protruded so as to be opposed to each other from a substantial center position of both closing sections,
wherein a ratio of an inner diameter in the vicinity of an end of said barrel section to an inner diameter of the center of the barrel section is equal to or greater than 0.8 and is less than 1.
By doing this, there is no need that a distance between an electric discharge space corner and a center is extended longer than necessary. Thus, the coolest point temperature does not fall more than necessary, and the light emission characteristics such as efficiency are improved.
According to a third aspect of the present invention, there is provided a ceramic envelope for high intensity discharge lamp made of a light transmission ceramics, said envelope comprising:
a cylindrical barrel section forming an electric discharge light emitting space;
an annular closing section that closes both ends of the barrel section, respectively;
a capillary section for inserting and fixing an electric discharge electrode to be outwardly protruded so as to be opposed to each other from a substantial center position of both closing sections,
wherein a surface roughness Ra of the interior surface of said barrel section is 0.01 xcexcm to 0.4 xcexcm, and the additive concentration in the vicinity of the interior surface of said barrel section is xc2xd or less of that in the vicinity of the center of the thickness.
By doing this, the light transmission of the barrel section can be improved, reaction with halide is restrained, and good electric discharge characteristics can be maintained. In particular, the surface roughness Ra is preferably from 0.01 xcexcm to 0.1 xcexcm.
According to a fourth aspect of the present invention, there is provided a ceramic envelope for high intensity discharge lamp made of a light transmission ceramics, said envelope comprising:
a cylindrical barrel section forming an electric discharge light emitting space;
an annular closing section that closes both ends of the barrel section, respectively;
a capillary section for inserting and fixing an electric discharge electrode to be outwardly protruded so as to be opposed to each other from a substantial center position of both closing sections,
wherein the barrel section thickness of at least one of the boundary sections between both of the barrel section and closing section is continuously increased at a ratio from 1.2 to 2.0 relevant to the thickness in the vicinity of the center of an electric discharge light emitting space, and a ratio of a diameter in the vicinity of an end of the barrel section to a diameter of the center of the barrel section is equal to or greater than 0.8, and is less than 1.0.
With this construction, even if a light emitting substance accumulates at an electric discharge light emitting space corner, a time required predetermined amount of corrosion reduction can be prolonged.
In addition, there is no need that a distance between an electric discharge space corner and a center is extended longer than necessary. Thus, lowering the coolest point temperature more than necessary does not occur, and the service life of the lamp can be further extended. Further, the light emission characteristics such as efficiency are improved.
According to a fifth aspect of the present invention, there is provided a ceramic envelope for high intensity discharge lamp as described above, wherein the surface roughness Ra of the anterior surface of the barrel section is from 0.01 xcexcm to 0.4 xcexcm, and the additive concentration of the surface of said barrel section is xc2xd or less of that in the vicinity of the center of the thickness.
According to a sixth aspect of the present invention, there is provided a ceramic envelope for high intensity discharge lamp, wherein an additive consists of at least one or more kinds of ScO3, MgO, ZrO2, Y2O3, and lanthanoid based rare earth oxide.
By doing this, abnormal grain growth of a ceramics base phase represented by alumina can be restrained, and uniform grain growth can be produced.